Power and free conveyor systems for moving bulky items through a manufacturing or assembly plant are well known. Such power and free conveyors include a "power" and a "free" conveyor track, generally disposed vertically with respect to each other. Operating within the power track is an endless drive chain with drive dogs periodically attached to the chain and extending toward the free track. These drive dogs are oriented to engage a trolley dog or actuator on a drive trolley operating within the free track. While the drive dogs are generally fixed in position relative to the drive chain, the trolley dogs on the drive trolleys are typically selectively retractable.
The vertically spaced free track generally follows the same path as the power track(s). As originally implemented, power and free conveyor systems were suspension systems with loads suspended from trolleys or carriers operating in the free track and with the power track disposed above the free track. These suspension systems have reached a high degree of sophistication and can include features such as the ability to stop and accumulate free trolleys in specific accumulating areas and transfer zones which include intersections where loads can be transferred between non-synchronous conveyor systems.
More recently, in response to the specific requirements of the automobile industry, floor mounted or "inverted" power and free systems have been developed. In these inverted systems, the power track and the free track are disposed beneath the floor of the factory, with the free track positioned above the power track. A plurality of trolleys operate within the free track, with certain of the trolleys being drive trolleys. Each drive trolley is connected to a load carriage. Such load carriages often include support wheels which roll along the factory floor to provide additional support and stability to the carriage. These are generally known as "dolly-type" carriers.
These inverted systems have the capability of handling bulkier and heavier loads, such as automobile chassis, while minimizing many dangerous conditions found in suspension systems. For example, inverted systems allow workers to safely climb on and off of the load carriages and they eliminate the danger inherent in the swinging loads of suspension systems.
At the same time, the development of inverted systems has presented a new and unique series of problems to designers. In a large factory, a single power and free conveyor can run for a mile or more. Within the length of the conveyor are a myriad of different assembly stations, many of which operate at different speeds, and thus require different drive chains. A longstanding problem in the design of power and free conveyors is the selective switching of the path of drive trolleys and their connected load carriages between, for example, an active conveyor loop and an accumulating conveyor loop.
Normally such switching is done via a pneumatically operated switch which swings a diverter between two positions. In a first diverter position, trolleys operating within the approaching active conveyor free track loop are allowed to continue on the same free track loop through the switch, thus remaining on the active conveyor. In the second position, the diverter deflects a trolley from the active conveyor free track loop onto the accumulating conveyor free track loop where it is engaged by drive members in the accumulating conveyor power track. When the drive trolley is diverted onto the accumulating conveyor, the towed dolly-type carrier is also diverted. However, as the active conveyor loop and the accumulating conveyor loop diverge (or converge) near the switch, the slots which extend through the factory floor in the active conveyor and the accumulating conveyor also diverge (or converge). This presents a problem, as the support wheels on one side of a passing dolly-type carriage must cross over the slot of the diverging (or converging), non-switched conveyor. The conveyor slots represent a substantial obstacle to the passage of the dolly-type carriage. At best, passage of the wheels of heavy load carriages over the slots, which are generally sealed with a resilient seal, causes the seal to be rapidly degraded, and jostles the load carriage, and the load it is carrying. At worst, the load carriage or the load can be upset as the carriage wheels encounter the slot.
It is clear then, that a need exists for a crossover seal selectively sealing the slot in one of two conveyor loops of a power and free conveyor as they diverge (or converge) near a switch. The crossover seal should automatically seal a crossover portion of a slot in the non-switched conveyor loop to allow dolly-type load carriages to traverse the switch without their wheels hitting the conveyor slot of the diverging (or converging), non-switched conveyor loop.